The present invention relates to magnetic memory, and more particularly, to a magnetic memory cell structure and a method for manufacturing the same.
A resistance-based memory device normally comprises an array of memory cells, each of which includes a memory element and a selection element coupled in series between two electrodes. The selection element functions like a switch to direct current or voltage through the selected memory element coupled thereto. The selection element may be a three-terminal device, such as transistor, or a two-terminal device, such as diode or Ovonic threshold switch. Upon application of an appropriate voltage or current to the selected memory element, the electrical property of the memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 is a schematic circuit diagram of a memory array 30, which comprises a plurality of memory cells 32 with each of the memory cells 32 including a two-terminal bi-directional selector 34 coupled to a resistance-based memory element 36 in series; a first plurality of parallel conductive lines 38 with each being coupled to a respective row of the two-terminal bi-directional selectors 34 in a first direction; and a second plurality of parallel conductive lines 40 with each being coupled to a respective row of the memory elements 36 in a second direction substantially perpendicular to the first direction. Accordingly, the memory cells 32 are located at the cross points between the first and second plurality of conductive lines 38 and 40. The first and second plurality of conductive lines 38 and 40 may be bit lines and word lines, respectively, or vice versa. Multiple layers of the memory array 30 may be stacked to form a monolithic three-dimensional memory device.
The resistance-based memory element 36 may be classified into at least one of several known groups based on their resistance switching mechanism. The memory element of Phase Change Random Access Memory (PCRAM) may comprise a phase change chalcogenide compound, which can switch between a resistive phase (amorphous or crystalline) and a conductive crystalline phase. The memory element of Conductive Bridging Random Access Memory (CBRAM) relies on the statistical bridging of metal rich precipitates therein for its switching mechanism. The memory element of CBRAM normally comprises a nominally insulating metal oxide material, which can switch to a lower electrical resistance state as the metal rich precipitates grow and link to form conductive paths upon application of an appropriate voltage. The memory element of Magnetic Random Access Memory (MRAM) typically comprises at least two layers of ferromagnetic materials with an insulating tunnel junction layer interposed therebetween. When a switching current is applied to the memory element of an MRAM device, one of the ferromagnetic layers will switch its magnetization direction with respect to that of the other magnetic layer, thereby changing the electrical resistance of the element.
A magnetic memory element normally includes a magnetic reference layer and a magnetic free layer with an electron tunnel junction layer interposed therebetween. The magnetic reference layer, the electron tunnel junction layer, and the magnetic free layer collectively form a magnetic tunneling junction (MTJ). Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The electron tunnel junction layer is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel or oriented in opposite directions. The stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer between parallel and anti-parallel with respect to the magnetization direction of the reference layer. Therefore, the MTJ has two stable resistance states that allow the MTJ to serve as a non-volatile memory element.
Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the same layers, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer planes.
The two-terminal bi-directional selector 34 normally includes two electrodes with a switching layer interposed therebetween. The switching layer is insulative in the absence of an applied voltage or voltage bias to the selector 34. When a sufficiently high voltage is applied to the selector 34, however, the switching layer becomes conductive and thus allows current to flow therethrough.
FIG. 2 illustrates the formation of a magnetic memory cell 32 by a conventional manufacturing method. The magnetic memory cell 32 includes a selector 34 and a magnetic memory element 36 coupled in series. The selector 34 includes a top electrode 42 and a bottom electrode 44 with a switching layer 46 interposed therebetween. The magnetic memory element 36 includes a magnetic tunnel junction (MTJ) structure 48 between an optional seed layer 50 and an optional cap layer 52. The selector 34 is formed on top of the magnetic memory element 36, which is formed on top of the first conductive line 40 (bit or word line).
With continuing reference to FIG. 2, the fabrication of the magnetic memory cell 32 begins by depositing all relevant films of the magnetic memory element 36 and the selector 34 on a planarized substrate containing therein the first conductive line 40. The pillar shaped magnetic memory cell 32 is then formed by etching the relevant films with an etch mask 52 thereon. The etching process can carried out by plasma etching or ion beam etching. Because of the weak etching resistance of the switching layer 46 compared with other layers in the magnetic memory cell 32 and the prolonged exposure of the switching layer 46 to the etching environment, the sidewall of the switching layer 46 tend to recede from the sidewalls of the adjacent layers, thereby creating a circumferential notch or cavity on the pillar shaped magnetic memory cell 32. During the etching process, re-deposited materials, such as magnetic materials in the MTJ structure 48 and noble metals in the top and bottom electrodes 42 and 44 that cannot be readily volatized, accumulate in the circumferential notch, thereby shunting the selector 34.
For the foregoing reasons, there is a need for a manufacturing method that can reliably produce magnetic memory cells with high yield.